1. Field of the Invention
The present invention relates generally to filter circuits having controllable frequency responses and, more particularly, to a filter circuit including an acoustic surface-wave filter device whose bandwidth can be controlled.
2. Description of the Prior Art
The capability of changing the bandwidth of a filter circuit finds many divers and useful applications. For example, in the field of radio broadcast reception, it is desirable to change the bandwidth of the IF (intermediate frequency) stage of an FM receiver to a relatively narrow band or to a relatively wide band depending upon the intensity of the received electric fields. This is because when an electric field is weak, the bandwidth is made narrow to enhance the signal-to-noise (S/N) ratio, sensitivity and selectivity; while when the electric field is strong, e.g., higher than a predetermined value, the bandwidth is made wide to enhance the distortion and stereo-separation characteristics.
There are, of course, various filter circuits which can be and have been used in the IF stage. It has been suggested that an acoustic surface-wave filter device may also be used. In general, an acoustic surface-wave filter device is comprised of input and output transducers formed on a body of piezo-electric material, each of the input and output transducers being made of a pair of interleaved electrodes and being separated by a predetermined distance so as to provide a predetermined delay characteristic or filter characteristic by propagating an acoustic surface-wave between the input and output transducers. The acoustic surface-wave filter offers superior group delay characteristics. Consequently, if used in the IF stage, the phase characteristic becomes flat, and accordingly the FM stereo-separation and distortion characteristics are improved. For these reasons, the acoustic surface-wave filter is most suitable for use in an FM receiver.
In order to change the bandwidth of such a filter, for example, to wide or narrow band characteristics, the number of conductive elements constituting the interleaved electrodes must be changed. However, when the number of conductive elements in the electrodes is changed, there is a resultant variation in the filter insertion loss and input and output impedances. Thus, the predictability of the band characteristics of the acoustic surface-wave filter device is difficult if these characteristics are to be changeable.